Hip prostheses are designed to restore the normal motion and stability of the hip joint when disease prevents the original, native joint from functioning without pain or functional limitation. Typically the artificial joint comprises the following:
1. A femoral component, typically fabricated from a high-strength metal alloy (e.g. cobalt-chromium, titanium-aluminum-vanadium or stainless-steel). Most commonly, the femoral component consists of: (i) a highly polished metal ball (termed the femoral head of the prosthesis), which is designed to articulate against a mating socket, and (ii) an elongated stem that is inserted into the intramedullary shaft of the femur where it is fixed to the bone to stabilize the femoral head during loading. The head of the prosthesis is mounted on a tapered spigot which is attached to an elongated stem via an angled segment termed the “neck” of the prosthesis. The neck lies outside the confines of the bony canal in which the stem is anchored and occupies the space between the pelvis and the femur.
2. A hemispherical socket, commonly referred to as an “acetabular cup.” This component is designed to articulate with the head of the femoral prosthesis. Acetabular cups are fabricated from a variety of materials, most commonly ultra high molecular weight (UHMW) polyethylene. Many designs consist of two elements: (i) a liner of wear-resistant polymeric or ceramic material, and (ii) a supporting metal shell, adapted for rigid fixation to the prepared surface of the acetabulum of the native pelvis.
Although it is intended that the prosthetic device will act like a normal joint in fully restoring the patient's range of motion and ease of movement, this goal is rarely achieved in practice. Most artificial hip prostheses allow the patient sufficient motion to perform many basic activities such as walking and sitting, but will not allow the patient to place his leg in more extreme positions accommodated by the normal joint. Although this is not a significant limitation when simple motions are involved (e.g. bending over and touching toes, or placing the thigh in contact with the chest), in many common activities of daily living, compound motions are involved. These motions require the femur to rotate about the hip joint in a plane that is not parallel or perpendicular to the front of the body. Common activities that necessitate compound rotations include rising from a low chair and picking up objects from the floor when seated. Other activities, such as crossing of the legs in a seated position or rolling over in bed, necessitate significant internal or external rotation of the femur about its longitudinal axis. In each of these situations, artificial hip joints typically allow significantly less motion than the normal joint, and attempts by the patient to force his hip to perform the activity will often cause dislocation of the joint, whereby the head of the femoral component is levered out of the acetabular cup. In most cases, the dislocated femoral head migrates to a position posterior to the pelvis with considerable pain and shortening of the limb.
The prevalence of dislocation after joint replacement is highly variable and is experienced by 0.7% to 5.5% of all patients after this surgical procedure. In patients who have to undergo implantation of a second artificial hip, the rate of dislocation is even greater, averaging between 5% and 20%.
The occurrence of dislocation is influenced by many variables, including the presence of additional disease processes, the laxity of the soft-tissue structures that surround the hip joint, and the alignment of the prosthetic components with respect to the femur and the pelvis. In the vast majority of cases, loss of stable articulation is preceded by a series of mechanical events that occur when the artificial joint is moved to the extremes of its arc of stable motion. During normal motion, the artificial hip approximates the motion of a ball-and-socket joint, with the femur moving about a point approximated by the center of rotation of the femoral head. If the femur is moved far enough from its initial, neutral position, mechanical contact or impingement will occur between either: a) the neck of the femoral stem and the edge of the bearing surface of the acetabular insert (referred to as “prosthetic impingement”), or b) points on the surface of the femur and the pelvis (referred to as “bony impingement”).
Prosthetic impingement is the most frequent source of mechanical limitation to the artificial hip at the extremes of its range of motion. Once this occurs, additional motion of the lower limb is only possible if the femur and the femoral component pivot about the point of impingement between the neck of the prosthesis, which is most frequently located at the junction between the anterior edge of the concave bearing surface and the outer face of the component. This pivoting motion causes the head of the prosthesis to lift out of its mating recess in the acetabular insert. Once more than half of the head is raised above the contact point between the head and the bearing surface, the resistance to dislodgement of the head from the socket is too low to resist the forces acting on the joint, thereby resulting in dislocation. The most common activities leading to neck impingement and hip dislocation are stooping, rising from a chair, pivoting on one leg, rolling over while lying in bed, and leg-crossing. In the majority of these positions, impingement occurs on the anterior/medial side of the neck. A chain of events similar to those observed after prosthetic impingement also accompanies joint motion after bony impingement; however, in this case the pivot point for raising the femoral head is the point of contact between the pelvis and the femur.
Previous inventors have recognized the importance of preventing or delaying impingement as much as possible to afford the maximum possible motion to the hip joint without the onset of subluxation and instability. Several different design modifications to the neck of the prosthesis and the acetabular insert have been proposed to increase the allowable range of motion of the artificial joint prior to impingement. Currently, modifications to the neck have concentrated on two primary strategies: 1) maximizing the ratio between the diameter of the femoral head and the diameter of the neck; and 2) modifications to the cross-sectional shape of the neck to provide more motion in positions where impingement is predicted to occur during activities that commonly associated with dislocation. It is well recognized in the art that impingement between the femoral neck and the acetabular insert commonly occurs at points located between about 10 and 20 mm below the head center along the neck axis. The exact location of contact varies with the femoral head diameter and the design of the acetabular cup. It is also recognized that the range of motion of the hip to impingement increases dramatically with increasing head diameter and decreasing neck diameter; however, the choice of head diameter is generally limited by the minimum thickness of the acetabular liner, which must generally exceed 5 mm to provide adequate wear resistance. Further, it is recognized that the range of motion of hip prostheses is greatly influenced by variations in the position of the femoral and acetabular components with respect to the skeleton. In practice, it is necessary to anticipate a range of these positions to ensure that any one design will be effective in reducing the incidence of dislocation in clinical practice.
It is generally recognized that attempts to improve the range of motion of the artificial joint by narrowing the neck of the prosthesis are inherently limited by the minimum strength needed to avoid mechanical failure of the device during service within the body. In practice, the neck of the prosthesis is designed to withstand a minimum repetitive load applied to the head of the implant. Most implants are designed with necks that will resist 1,200–1,700 pounds of loading for 10 million cycles, depending upon the weight and activity level of the intended patient population. For necks that are circular in cross-section, this requirement necessitates that the neck have a diameter of at least 9 mm at the level along the axis of the neck where prosthetic impingement normally occurs. The precise value of neck diameter that is required to provide sufficient strength to avoid mechanical failure during the service life of the prosthetic device is a function of many factors, most notably the fatigue strength of the alloy utilized to fabricate the prosthesis and the severity, direction, and duration of loads applied to the device.
Though most femoral prostheses have necks that are circular in transverse cross-section, those skilled in the art have developed necks with non-circular necks to enhance the balance between strength and the motion of the joint. Consequently, prostheses exist that have necks with transverse cross-sections that are either circular 50, rectangular 52, oval 51, or trapezoidal 54 (FIG. 4). Although these strategies do increase the range of motion of the joint, they also lead to a significant reduction in strength, particularly when the load applied to the head of the prosthesis has a substantial anterior-posterior component which occurs during several strenuous activities, such as walking up and down stairs or rising from a chair. The magnitude of the anterior/posterior bending moment severely limits the extent to which the anterior-posterior width of the neck may be reduced to improve motion and is a fundamental deficiency of some early trapezoidal designs. Consequently, modifications to the neck to increase range of motion must be offset by increasing all dimensions of the part to regain strength. This partly reduces the gains in motion derived from the original change in cross-sectional shape.
The present invention is directed to a femoral implant for use in hip arthroplasty whereby the neck portion is designed to provide an improved balance between the range of motion of the joint during common activities and the resistance of the device to mechanical failure through its neck portion. These benefits will be realized independent of the strength of the resulting neck; however, the absolute values of neck strength will naturally increase if the dimensions of the neck are increased, with concomitant reduction in joint motion. Conversely, reductions in the dimensions of the neck, while maintaining the transverse shape taught by the invention, will lead to increased absolute motion in combination with reduced mechanical strength. Specifically, the implant comprises a longitudinal stem having a distal end and a proximal end, the stem further having a longitudinal axis extending from the proximal end to the distal end. The implant includes a neck portion extending from the proximal end of the stem and a femoral head configured for engagement within an acetabulum. The neck portion further has an axis extending through the femoral head and neck portion and intersecting the stem axis. A transverse cross section of the neck portion, taken perpendicular to the neck axis, further comprises: (i) a medial/lateral axis bisecting the cross-section through a medial-most point along the medial edge and the lateral-most point along the lateral edge of the cross-section, thereby creating two anterior and posterior halves about the medial/lateral axis, further defining a maximum medial/lateral width of the cross-section; and (ii) an anterior/posterior axis, perpendicularly intersecting the medial/lateral axis. The transverse cross-section further has a configuration defined by a medially-positioned circle comprising: (i) the medial-most point of the cross section, (ii) a point taken along the anterior edge of the cross section, and (iii) a point taken along the posterior edge of the cross section, wherein each of the anterior and posterior edge points are located along the anterior and posterior edges, respectively, at a distance of 10% of the medial/lateral width measured laterally from the medial-most edge point. The medially-positioned circle further has a medial diameter ranging in length of about 66% or less of the maximum anterior/posterior width of the cross section. More preferably, the medial diameter of the circle is about 45–55%, most preferably about 50% of the maximum anterior/posterior width of the cross section. Finally, the transverse cross section may be taken at any point located at or between about 10 mm to about 22 mm below the center of the femoral head along the neck axis and maintain the described configuration.